Cryobiology of coral fragments

Evaluating the coral microbiome during cryopreservation

Coral reefs are threatened by various local and global stressors, including elevated ocean temperatures due to anthropogenic climate change. Coral cryopreservation could help secure the diversity of threatened corals. Recently, isochoric vitrification was used to demonstrate that coral fragments lived to 24 hr post-thaw; however, in this study, they were stressed post-thaw. The microbial portion of the coral holobiont has been shown to affect host fitness and the impact of cryopreservation treatment on coral microbiomes is unknown. Therefore, we examined the coral-associated bacterial communities pre- and post-cryopreservation treatments, with a view towards informing potential future stress reduction strategies. We characterized the microbiome of the Hawaiian finger coral, Porites compressa in the wild and at seven steps during the isochoric vitrification process. We observed significant changes in microbiome composition, including: 1) the natural wild microbiomes of P. compressa were dominated by Endozoicomonadaceae (76.5 % relative abundance) and consistent between samples, independent of collection location across Kāne'ohe Bay; 2) Endozoicomonadaceae were reduced to <6.9 % in captivity, and further reduced to <0.5 % relative abundance after isochoric vitrification; and 3) Vibrionaceae dominated communities post-thaw (58.5-74.7 % abundance). Thus, the capture and cryopreservation processes, are implicated as possible causal agents of dysbiosis characterized by the loss of putatively beneficial symbionts (Endozoicomonadaceae) and overgrowth of potential pathogens (Vibrionaceae). Offsetting these changes with probiotic restoration treatments may alleviate cryopreservation stress and improve post-thaw husbandry.

Multiomics data integration, limitations, and prospects to reveal the metabolic activity of the coral holobiont

Since their radiation in the Middle Triassic period ∼240 million years ago, stony corals have survived past climate fluctuations and five mass extinctions. Their long-term survival underscores the inherent resilience of corals, particularly when considering the nutrient-poor marine environments in which they have thrived. However, coral bleaching has emerged as a global threat to coral survival, requiring rapid advancements in coral research to understand holobiont stress responses and allow for interventions before extensive bleaching occurs. This review encompasses the potential, as well as the limits, of multiomics data applications when applied to the coral holobiont. Synopses for how different omics tools have been applied to date and their current restrictions are discussed, in addition to ways these restrictions may be overcome, such as recruiting new technology to studies, utilizing novel bioinformatics approaches, and generally integrating omics data. Lastly, this review presents considerations for the design of holobiont multiomics studies to support lab-to-field advancements of coral stress marker monitoring systems. Although much of the bleaching mechanism has eluded investigation to date, multiomic studies have already produced key findings regarding the holobiont's stress response, and have the potential to advance the field further.

Lipid profiling in chilled coral larvae

Coral reefs are disappearing worldwide as a result of several harmful human activities. The establishment of cryobanks can secure a future for these ecosystems. To design effective cryopreservation protocols, basic proprieties such as chilling tolerance and lipid content must be assessed. In the present study, we investigated chilling sensitivity and the effect of chilling exposure on the lipid content and composition of larvae belonging to 2 common Indo-Pacific corals: Seriatopora caliendrum and Pocillopora verrucosa. The viability of coral larvae incubated with 0.5, 1, and 2 M ethylene glycol (EG), propylene glycol (PG), dimethyl sulfoxide (Me₂SO), methanol, or glycerol and kept at 5 °C for different time periods was documented. In addition, we investigated the content of cholesterol, triacylglycerol (TAG), wax ester (WE), sterol ester (SE), lysophosphatidylcholine, phosphatidylcholine, phosphatidylethanolamine, and several fatty acid (FA) classes in coral propagules incubated with 1 M PG or EG and kept at 5 °C for 6 h. Moreover, we examined seasonal changes in the aforementioned lipid classes in coral larvae. S. caliendrum incubated with 0.5 M PG or Me₂SO and chilled for 2 h exhibited a viability rate of 11 ± 11%, whereas P. verrucosa exhibited a viability rate of 22 ± 14% after being chilled for 4 h. Furthermore, the results indicated that chilling exposure did not affect the content of any investigated lipid class in either species. The higher concentration of SE in P. verrucosa compared to S. caliendrum larvae may have contributed to the different cryotolerance displayed by the 2 larval species. A year-round lipid analysis of both coral larvae species revealed trends of homeoviscous adaptation and seasonal enhancement of lipid fluxes from symbionts to the host. During winter, the cholesterol/phospholipid ratio significantly increased, and P. verrucosa larvae exhibited an averagely decrease in FA chain lengths. During spring and summer, intracellular lipid content in the form of TAGs and WEs significantly increased in both species, and the average content of Symbiodiniaceae-derived FAs increased in P. verrucosa larvae. We concluded that the low cryotolerance displayed by S. caliendrum and P. verrucosa larvae is attributable to their chilling-sensitive membrane lipid profile and the high intracellular lipid content provided by their endosymbionts.

Cnidarian Cell Cryopreservation: A Powerful Tool for Cultivation and Functional Assays

Cnidarian primary cell cultures have a strong potential to become a universal tool to assess stress-response mechanisms at the cellular level. However, primary cell cultures are time-consuming regarding their establishment and maintenance. Cryopreservation is a commonly used approach to provide stable cell stocks for experiments, but it is yet to be established for Cnidarian cell cultures. The aim of this study was therefore to design a cryopreservation protocol for primary cell cultures of the Cnidarian Anemonia viridis, using dimethyl sulfoxide (DMSO) as a cryoprotectant, enriched or not with fetal bovine serum (FBS). We determined that DMSO 5% with 25% FBS was an efficient cryosolution, resulting in 70% of post-thaw cell survival. The success of this protocol was first confirmed by a constant post-thaw survival independently of the cell culture age (up to 45 days old) and the storage period (up to 87 days). Finally, cryopreserved cells displayed a long-term recovery with a maintenance of the primary cell culture parameters and cellular functions: formation of cell aggregates, high viability and constant cell growth, and unchanged intrinsic resistance to hyperthermal stress. These results will further bring new opportunities for the scientific community interested in molecular, cellular, and biochemical aspects of cnidarian biology.

Seasonal Preservation Success of the Marine Dinoflagellate Coral Symbiont, Symbiodinium sp

Coral reefs are some of the most diverse and productive ecosystems on the planet, but are threatened by global and local stressors, mandating the need for incorporating ex situ conservation practices. One approach that is highly protective is the development of genome resource banks that preserve the species and its genetic diversity. A critical component of the reef are the endosymbiotic algae, Symbiodinium sp., living within most coral that transfer energy-rich sugars to their hosts. Although Symbiodinium are maintained alive in culture collections around the world, the cryopreservation of these algae to prevent loss and genetic drift is not well-defined. This study examined the quantum yield physiology and freezing protocols that resulted in survival of Symbiodinium at 24 h post-thawing. Only the ultra-rapid procedure called vitrification resulted in success whereas conventional slow freezing protocols did not. We determined that success also depended on using a thin film of agar with embedded Symbiodinium on Cryotops, a process that yielded a post-thaw viability of >50% in extracted and vitrified Symbiodinium from Fungia scutaria, Pocillopora damicornis and Porites compressa. Additionally, there also was a seasonal influence on vitrification success as the best post-thaw survival of F. scutaria occurred in winter and spring compared to summer and fall (P < 0.05). These findings lay the foundation for developing a viable genome resource bank for the world’s Symbiodinium that, in turn, will not only protect this critical element of coral functionality but serve as a resource for understanding the complexities of symbiosis, support selective breeding experiments to develop more thermally resilient strains of coral, and provide a ‘gold-standard’ genomics collection, allowing for full genomic sequencing of unique Symbiodinium strains.

Towards a vitrification-based cryopreservation protocol for the coral Pocillopora damicornis L.: Tolerance of tissue balls to 4.5 M cryoprotectant solutions

In this study, we tested the tolerance of tissue balls (TBs, 100-400 μm in diameter) from the coral Pocillopora damicornis produced using mechanical excision to exposure to cryoprotectant (CPA) solutions. TBs were treated for 20 min at room temperature with individual, binary, ternary or quaternary CPA solutions with a total molarity from 2.0 to 5.0M. Four CPAs were used: ethylene glycol (EG), dimethylsulfoxide (Me2SO), methanol (Met) and glycerol (Gly). In some experiments, the molarity of the CPA solutions was increased and decreased in a stepwise manner. The tolerance of TBs following CPA treatment was evaluated using two parameters. The Tissue Ball Regression (expressed in μm/h) measured the diameter regression of TBs over time. The % Undamaged TBs quantified the proportion of TBs, which remained intact over time after the CPA treatment. TBs tolerated exposure to binary solutions with a total molarity of 4.0 M containing 2.0 M EG+2.0 M Met and 2.0 MEG+2.0 M Gly. TBs displayed tolerance to ternary solutions with a total molarity up to 3.0 M, containing each CPA at 1.0 M. Quaternary solutions with a total molarity of 4.0M containing each CPA at 1.0 M were not tolerated by TBs. When the molarity of the CPA solutions was increased and decreased in a stepwise manner, TBs withstood exposure to a CPA solution with a total molarity of 4.5 M, containing 1.5 M EG+1.5 M Gly+1.5 M Me(2)SO. This study confirmed the interest of using TBs to test CPA solutions, with the objective of developing a vitrification-based cryopreservation protocol.

Survival of tissue balls from the coral Pocillopora damicornis L. exposed to cryoprotectant solutions

In this study, the tolerance of tissue balls (TBs, 100-300 μm in diameter) from the coral Pocillopora damicornis produced using mechanical excision to exposure to cryoprotectant (CPA) solutions was tested. TBs were treated for 20 min at room temperature with solutions of ethylene glycol (EG), methanol (Met), glycerol (Gly) or dimethyl sulfoxide (Me2SO) at concentrations between 1.0 and 4.5M. Two parameters were used to evaluate the survival of TBs following CPA treatment. The Undamaged Duration of Tissue Balls (expressed in h) corresponded to the time period during which the membrane surface of TBs remained smooth and their motility was preserved. Tissue Ball Regression (expressed in μm/h) corresponded to the size reduction of TBs over time. TBs tolerated exposure to all CPAs tested at the three lower concentrations employed (1.0 M, 1.5 M and 2.0 M). No survival was achieved following exposure to a 4.5 M CPA solution. At concentrations of 3.0 and 4.0 M, higher Undamaged Duration of Tissue Balls and lower Tissue Ball Regression were obtained following treatment with EG compared to the other three CPAs. Our experiments show that TBs constitute a good experimental material to evaluate CPA toxicity on corals using large numbers of samples. Performing preliminary experiments with TBs may allow reducing the number of tests carried out with less easily available coral forms such as planulae, thereby preserving larval stocks.